Kraft pulp yield by heat treatment of polysulphide liquors generated by oxidation

ABSTRACT

Oxidized white liquor is heat treated to increase the concentration of PS UV  measured at 285 or 286 nm or PS VIS  measured at 416 nm and the PS UV /PS GR  or PS VIS /PS GR  ratio, whereby the content of active polysulphide in the total polysulphide is increased which active polysulphide can be exploited to increase pulp yield in Kraft pulping.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This Application claims priority under 35 U.S.C. 119(e) from U.S.Provisional Application Ser. No. 60/263,519, filed Jan. 24, 2001.

BACKGROUND OF THE INVENTION

[0002] (i) Field of the Invention

[0003] The present invention relates to an improvement in any processwhich generates polysulphide by the oxidation of white liquor theinvention also relates to a method of increasing the yield of pulp inKraft pulping with an oxidized white liquor.

[0004] (ii) Description of the Prior Art

[0005] In Kraft pulping operations, where the goal is to remove ligninwhile retaining carbohydrates, yield is increased by minimizingcarbohydrate (i.e., cellulose and hemicellulose) degradation. Thisdegradation occurs through the “peeling” reaction in which sugar unitsare sequentially removed from the reducing end group of thepolysaccharide chains. One way to prevent this reaction is to convertaldehyde groups on the wood polysaccharides to a form which isrelatively inert to further “peeling”. This conversion is achieved byeither oxidizing the aldehyde to its corresponding carboxylic acid(Alfredsson et al., 1963), (Holton, 1977) or, alternatively, reducing itto its alcohol form (Hartler, 1959), (Pettersson et al., 1961). The twomethods that are applied in the pulp and paper industry involve theoxidation process and use anthraquinone (Holton, 1977), or polysulphide(Clayton et al., 1967), (Landmark et al., 1965), (Sanyer et al., 1964),(Teder, 1969), or both as oxidizing agents. Anthraquinone is a catalyticadditive while polysulphide is generated from white liquor by oxidationof sodium sulfide in one of several processes (Dorris, 1992), (Smith etal., 1977).

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to increase theconcentration of PS_(UV) measured at 285 or 286 nm or PS_(VIS) measuredat 416 nm and the PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio ofpolysulphide liquors generated by the oxidation of white liquor withoutsubstantial loss of polysulphide charge.

[0007] It is a further object of the present invention to increase theyield of pulp from wood particles by cooking the wood particles in apolysulphide liquor having a high concentration of PS_(UV) measured at285 or 286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio, generated by the oxidation of white liquor.

[0008] It is a still further object of the invention to increase thecontent of active polysulphide in a polysulphide liquor generated by theoxidation of white liquor.

[0009] It is another object of the present invention to increase theyield of pulp from wood particles by cooking wood particles in apolysulphide liquor generated by the oxidation of white liquors andwhich have been thermally treated.

[0010] In accordance with one aspect of the invention, there is provideda method which comprises exposing an oxidized white liquor to atemperature effective to increase the concentration of PS_(UV) measuredat 285 or 286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR)or PS_(VIS)/PS_(GR) ratio of polysulphide in the oxidized white liquor.

[0011] In accordance with another aspect of the invention, there isprovided a method of increasing the concentration of PS_(UV) measured at285 or 286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio of a polysulphide liquor, said method comprisingheating or cooling if necessary and then storing said liquor at atemperature between 20° C. and 95° C. for a time of up to 72 hours.

[0012] In accordance with yet another aspect of the invention, there isprovided a method of producing an oxidized white liquor containingpolysulphide comprising: oxidizing a white liquor to produce an oxidizedwhite liquor containing polysulphide and having a first PS_(UV)/PS_(GR)or PS_(VIS)/PS_(GR) ratio, and heat said oxidized white liquor toproduce an oxidized white liquor having a second PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio, wherein said second ratio is greater than saidfirst ratio.

[0013] In accordance with still another aspect of the invention, thereis provided a method of increasing the yield of pulp in Kraft pulpingwith a white liquor containing polysulphide comprising: oxidizing awhite liquor to produce an oxidized white liquor containingpolysulphide, ii) heating or cooling if necessary and then storing saidoxidized white liquor to increase the concentration of PS_(UV) measuredat 285 or 286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR)or PS_(VIS)/PS_(GR) ratio of polysulphide in the oxidized white liquor,and in a subsequent step: iii) delignifying pulp with the oxidized whiteliquor from step ii).

[0014] The invention relates to the heat treatment of a polysulphideliquor generated by the oxidation of white liquor. This heat treatmentis preferably at a temperature below 95° C., more preferably between 20°C. and 95° C., and most preferably between 50 and 95° C. for a time upto 72 hours, preferably 1 to 48 hours, more preferably 6 to 30 hours,even more preferably 12 to 24 hours.

[0015] In one preferred embodiment of the invention, there is provided amethod which comprises exposing an oxidized white liquor produced byoxidation of white liquor in the presence of lime mud, or MnO₂ or bothlime and MnO₂ to a temperature effective to increase the concentrationof PS_(UV) measured at 285 or 286 nm or PS_(VIS) measured at 416 nm andthe PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of polysulphide in theoxidized white liquor. Preferably the temperature, in this latterpreferred embodiment, is below 95° C. and the period of exposure is fora time up to 72 hours; more preferably the temperature is between 50° C.and 95° C., for an exposure time up to 48 hours.

[0016] In another preferred embodiment of the invention, there isprovided a method of increasing the PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR)ratio of a polysulphide liquor generated by oxidation of white liquor inthe presence of lime mud or MnO₂ or both lime mud and MnO₂;which methodcomprises heat treating the oxidized white liquor at a temperaturebetween 50° C. and 95° C. for a time of up to 72 hours, and preferablyup to 48 hours.

[0017] In still another preferred embodiment of the invention, there isprovided a method of increasing the yield of pulp in Kraft pulping witha white liquor containing polysulphide comprising: i) oxidizing a whiteliquor in the presence of lime mud, MnO₂ or both lime mud and MnO₂ toproduce an oxidized white liquor containing polysulphide, ii) heattreating the oxidized white liquor to increase the concentration ofPS_(UV) measured at 285 or 286 nm or PS_(VIS) measured at 416 nm and thePS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of polysulphide in theoxidized white liquor, and iii) cooking wood chips with the oxidizedwhite liquor from step ii) to produce pulp.

BRIEF DESCRIPTION OF DRAWINGS

[0018]FIG. 1 illustrates graphically the yield benefit in pulpdelignification with increase in PS_(UV)/PS_(GR) ratio;

[0019]FIG. 2 illustrates graphically the relationship betweenpolysulphide decomposition and temperature; and

[0020]FIGS. 3 and 4 show the relationship between pulp yield andpermanganate number for polysulphide liquors of the invention atdifferent temperatures; and conventional white liquors for differentpulps.

DETAILED DESCRIPTION OF THE INVENTION

[0021] i) Oxidized White Liquor

[0022] The oxidized white liquor in this invention is one produced byoxidizing sodium sulphide in the white liquor to sodium polysulphide.

[0023] The invention is not confined to any particular oxidationprocedure for producing the polysulphide or oxidized white liquor. Theinvention thus extends to oxidized white liquors in which the oxidationis carried out with oxygen or oxygen-containing gases such as air, inthe presence of a catalyst, for example, wet-proofed activated carbon inthe MOXY (trademark of The Mead Corporation) process; lime mud in thePAPRILOX (trademark of Pulp and Paper Research Institute of Canada)process, lime mud spiked with manganese dioxide; or with oxygen oroxygen containing gas in the presence of a metal oxide, such as oxidesof manganese, iron, cobalt, zinc, aluminum, nickel or chromium, whichmetal oxide functions as a catalyst for polysulphide formation.

[0024] An especially preferred or advantageous oxidized white liquor foruse in the invention is that produced by oxidation of a white liquorproduced by causticizing green liquor and containing the lime mudgenerated in the causticization process.

[0025] Green liquor is produced from the smelt derived from black liquorin chemical recovery of a conventional Kraft liquor cycle. The greenliquor comprises sodium carbonate and sodium sulphide and thecausticization involves addition of lime, calcium oxide, to the greenliquid. The lime reacts with the sodium carbonate to produce sodiumhydroxide with precipitation of insoluble calcium carbonate. Thesuspended solids comprising the calcium carbonate, unreacted calciumoxide and other insoluble solids present in the smelt, is referred to aslime mud.

[0026] This lime mud is thus a by-product of the white liquor formation.

[0027] The white liquor suspension containing lime mud, can be employeddirectly in the production of the oxidized white liquor, as outlined inU.S. Pat. No. 5,082,526 incorporated herein by reference. Especiallyadvantageously, a catalytic amount of manganese dioxide is added to thewhite liquor suspension to further enhance the oxidation. Suitablecatalytic amounts of manganese dioxide comprise 0.1 to 2.0 g/l of awhite liquor.

[0028] In such case, the resulting oxidized white liquor contains thelime mud and, where applicable, the added manganese dioxide catalyst, assuspended solids.

[0029] ii. Heat Treatment

[0030] The process of this invention for increasing the concentration ofPS_(UV) measured at 285 or 286 nm or PS_(VIS) measured at 416 nm and thePS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of polysulphide liquorsgenerated by the oxidation of white liquor is a process in which thepolysulphide liquor is heat treated within a range of temperatures andtimes without substantial loss of polysulphide charge.

[0031] Increasing the concentration of PS_(UV) measured at 285 or 286 nmor PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio increases the active polysulphide content in theliquor. The oxidized white liquor is separated from the oxidationcatalysts such as lime mud and manganese oxide prior to the heattreatment.

[0032] In particularly advantageous embodiments, the liquors generatedby the oxidation of white liquor in the presence of MnO₂′lime mud orboth MnO₂ and lime mud are heat treated at a temperature below 95° C.for a time up to 72 hours to increase the concentration of PS_(UV)measured at 285 or 286 nm or PS_(VIS) measured at 416 nm and thePS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio.

[0033] The temperature of the polysulphide liquor for the heat treatmentcan be adjusted using a heat exchanger. The temperature of thepolysulphide liquor can also be adjusted by evaporative cooling with anoxygen-containing gas. Normal practice is to remove the heat generatedby the reactions between sodium sulphide and oxygen, to prevent theoxidized liquor temperature from rising to or above the liquor boilingpoint (Uloth et al., 1997, Tench et al., 1999). Storage may be providedby existing tankage provided both for liquor clarification and flowbuffering or by new tankage. The target storage temperature and storagetime can be optimized to ensure that the maximum charge of activepolysulphide in the polysulphide liquor, is delivered to the pulpdigester.

[0034] The heat treatment is preferably carried out by maintaining theoxidized liquor at a temperature of 50° C. to 90° C. for a time of 1 to48 hours.

[0035] iii) Polysulphide PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) Ratio

[0036] Polysulphide can be generated from sodium sulphide in a whiteliquor by various methods including the direct addition of sulphur tothe white liquor. However, this method cannot be used industriallywithout a bleed of sulphur from the Kraft recovery cycle, which isexpensive to provide. Having different methods of polysulphidegeneration, however, allows comparisons of the form of the polysulphidethat is generated by each of the different methods. These comparisonshave shown that there are differences in what is measured aspolysulphide when polysulphide liquor is generated by the directaddition of sulphur to the white liquor and when it is generated by theoxidation of white liquor.

[0037] Polysulphide can be measured in many ways but two of the mostsimple and effective are measurement by gravimetry (PS_(GR)) and by UVor VIS absorption (PS_(UV) or PS_(VIS)). The two methods can be used togive a PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio. Polysulphide generatedby direct sulphur addition has a PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR)ratio very close to 1, but polysulphide generated by oxidation of whiteliquor has a ratio that varies depending on the way that it has beenmade.

[0038] The differences in the form of polysulphide in the white liquorchange the degree to which the yield of pulp is increased by theapplication of a given polysulphide charge. Polysulphide liquors thathave a PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of 1 deliver the fullyield expected from the application of a given polysulphide charge inthe Kraft pulping process. In such polysulphide liquors, thepolysulphide content may thus be considered to be active polysulphidei.e. polysulphide which oxidizes aldehyde groups on wood polysaccharidesto inhibit carbohydrate degradation during delignification of pulp.

[0039] Oxidized liquors that have lower ratios are found to deliverdiminishing amounts of the expected yield (FIG. 1).

[0040] The lower the PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio, thelower is the content of active polysulphide in the polysulphide of theliquor, and conversely, the higher is the content of inactivepolysulphide.

[0041] It is therefore desirable in industrial application that theconcentration of PS_(UV) measured at 285 or 286 nm or PS_(VIS) measuredat 416 nm and the PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of thepolysulphide liquor be as high as possible, or as close to 1 aspossible.

[0042] The heat treatment of the invention does result in some loss inthe total polysulphide content, determined as both active and inactive.The loss depends on the treatment temperature and time.

[0043] It will be recognized that the heat treatment parameters aredesirably selected to establish a satisfactory content of activepolysulphide for the protective oxidation of the carbohydrate aldehydegroups. As such, a balance is to be achieved between PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio and the actual concentration of activepolysulphide. A PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio close to 1will not be beneficial if the heat treatment has decreased the totalpolysulphide content to a level where the active polysulphide content isinadequate for the protective oxidation reaction.

[0044] On the other hand, a low PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR)ratio will be beneficial where the total polysulphide content remainshigh such that the ratio signifies an adequate active polysulphidecontent for the protective oxidation reaction.

[0045] In general, an active polysulphide concentration of at least 4g/l, and preferably at least 6 g/l, in the oxidized white liquor, isrequired for effective oxidation of the carbohydrate aldehyde groups inthe wood chips.

[0046] The PS_(UV) can be measured at, for example, at 285 nm, 286 nm ora 416 nm wavelength.

[0047] Experimental

[0048] The liquors used to generate the results in FIG. 1 were obtainedas follows. White liquors of varying concentrations and compositions(800 mL; preheated to 70° C. in a microwave oven) were brought to 90° C.(in an oil bath) in a stainless steel reactor (1 L) equipped with acondenser (5° C.) and ports for adding gas and MnO₂ and for withdrawingsamples. During the rise to temperature, the liquor was stirredmechanically (600 rpm; Eurostar Power Digi-Visc-trademark) under anitrogen flow (50 mL/min; 2 μm stainless steel sparger(Supelco-trademark)). The impeller type used was a Rushton disk turbinewith 6 flat blades (48 mm diameter). Finely powdered MnO₂ (1 g/L;Brickox 6807-trademark of Prince Manufacturing Company) was added to thereactor when the liquor reached 90° C. PS_(OWL) was generated bybubbling air (450 mL/min) into the liquor at a constant stirring rate of1000 rpm. The generation of polysulphide was monitored with a UVspectrometer at 286 nm (HP Vectra QS/165-trademark; 1 mm path-lengthcells; oxygen-free NaOH (1N) used for dilution and blanks). Anabsorptivity of 43.48 Lg⁻¹cm⁻¹ was used to calculate the concentrationof PS_(UV). Samples were removed from the reactor using a plasticsyringe, immediately filtered on a ceramic Buchner funnel, and stored inpolyethylene containers under argon prior to their analysis. Thestainless steel sparger was cleaned with HCl (3 N) prior to subsequentuse. The PS_(GR) charge used in all the pulping experiments was 1.58% onwood (oven dried basis).

[0049] The PS_(UV) measurement for the ratios shown in FIG. 1 wereobtained at 286 nm, a wavelength at which all polysulphide speciesabsorb with the same absorptivity. The PS measurement, however, couldalso have been made at 416 nm with a similar relationship betweenPS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio and yield being found.

[0050]FIG. 2 presents the % decomposition of polysulphide determined bygravimetry and by UV spectrophotometry (286 nm) of a typical liquorproduced by catalytic oxidation with manganese dioxide as it is heatedat 1.8° C. per minute; this corresponds to the conventional rise totemperature used in Kraft cooking of 90 min to 170° C. It is very clearfrom this figure that, at temperatures above 100° C., polysulphidedecomposes rapidly.

[0051] Similar results can be generated by using liquors that areproduced by oxidizing white liquor with air in the presence of awet-proofed activated carbon catalyst. At the industrial scale, this isdone in a single oxidation step with compressed air blown into a fixedbed of the carbon black catalyst. Smith and Sanders (U.S. Pat. No.4,024,229) have presented some details on the production of PTFE-coatedcatalyst. Industrially the MOXY (trademark of The Mead Corporation) andChiyoda processes both use carbon as the oxidation catalyst Theseliquors also produce liquors that have PS_(UV)/PS_(GR) ratios of lessthan 1 and which can be improved by the heat treatment of the invention.Heat treatment increases the concentration of PS_(UV) measured at 285 or286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio of these liquors and therefore, the fibre yieldwhen using them for pulping.

EXAMPLES Example 1

[0052] Oxidized white liquor was produced by causticizing 0.75 L ofgreen liquor with 45 g reburned lime, spiked with 0.6 g MnO₂. Themanganese content of clarified green liquor samples is typically 0.3 to6.0 mg/L. The amount of manganese added in the MnO₂ in this example (504mg/L) is about a hundred times that normally found in green liquor.After 100 min causticizing time at 90° C., oxygen was sparged into thecausticized slurry at a rate of 0.1 L/min for 30 min. After oxidation,the resulting CaCO₃ lime mud with added MnO₂, was separated from theoxidized white liquor. Samples of the clarified white liquor were thenstored in a thermostated bath held at a desired temperature. At regulartime intervals, small samples of liquor were withdrawn for determinationof polysulphide concentration by UV spectrometry (PS_(UV)) and bygravimetry (PS_(GR)).

[0053] Example 1 illustrates the increase in the PS_(UV/PS) _(GR) ratiowhen a freshly oxidized white liquor is treated at 73° C. for up to 48hours.

[0054] Table I shows that by heat treatment at 73° C. for 48 hours, thePS_(UV)/PS_(GR) ratio was changed from 0.45 to 0.86 while thepolysulphide concentration (PS_(GR)) was only decreased from 9.3 g/L to7.1 g/L. Over 48 hours at 73 C, the PS_(UV) concentration increased by44%, from 4.23 to 6.08 gpl as sulphur. As can be seen from FIG. 1, suchan increase in ratio will allow the yield increase from a givenconcentration of polysulphide to be increased from zero to almost thefull potential of that concentration. TABLE 1 Stability at 73° C. PS PSconcentration concentration Sample 286 (UV) (gravimetry) PS_(UV)/PS_(GR)Time (h) abs g/L g/L) Ratio 0 0.73 4.23 9.34 0.45 1 0.78 4.54 8.12 0.562 0.81 4.71 8.36 0.56 3 0.85 4.92 8.30 0.59 20 1.05 6.08 7.06 0.86 241.06 6.13 7.60 0.81 48 1.05 6.08 7.10 0.86

Example 2

[0055] Example 2 illustrates the change in the ratio when the sameliquor is heat treated at 95° C. At this temperature the activation ofthe liquor, as measured by the change in ratio, is very rapid. Within anhour the ratio has increased to a useful 0.74. Long times of treatment(>3 hours) are less useful at this temperature because of the increasingloss of polysulphide concentration measured either by UV spectrometry orgravimetry. TABLE 2 Stability at 95° C. PS con- PS con- concentrationconcentration Sample 286 (UV) (gravimetry) PS_(UV)/PS_(GR) Time (h) absg/L g/L Ratio 0 0.73 4.23 9.34 0.45 1 0.80 4.65 6.32 0.74 2 0.83 4.815.86 0.82 3 0.85 4.91 5.36 0.92 20 0.71 4.11 5.00 0.82 24 0.70 4.07 5.300.77 48 0.57 3.31 4.20 0.79

Example 3

[0056] Example 3 illustrates the change in the ratio and polysulphideconcentration at an intermediate temperature of 85° C. At thistemperature, it takes between 2 and 3 hours for the activation of theliquor. Again longer times of treatment are less useful because of theincreasing loss of polysulphide charge. TABLE 3 Stability at 85° C. PScon- PS con- concentration concentration Sample 286 (UV) (gravimetry)PS_(UV)/PS_(GR) Time (h) abs g/L g/L Ratio 0 0.78 4.52 9.78 0.46 0.830.82 4.74 8.08 0.59 2 0.83 4.80 6.98 0.69 3 0.89 5.14 6.32 0.81 4 0.864.99 6.16 0.81 10 0.73 4.22 5.18 0.81 22 0.74 4.28 4.66 0.92 26 0.663.85 4.28 0.90 50.42 0.53 3.05 3.70 0.82

Example 4

[0057] Example 4 illustrates the change in the ratio and polysulphidecharge at 78° C. but with the liquor having been pre-activated over 4days at ambient temperature. The ambient temperature treatment increasedthe ratio from 0.46 to 0.55 without any loss of polysulphideconcentration. TABLE 4 Stability at 78° C. PS con- PS con- concentrationconcentration Sample 286 (UV) (gravimetry) PS_(UV)/PS_(GR) Time (h) absg/L g/L Ratio 0 0.92 5.33 9.62 0.55 0.83 0.87 5.02 8.86 0.57 2.5 0.875.05 7.86 0.64 3.5 0.87 5.02 7.78 0.65 4.5 0.88 5.08 6.56 0.77 10 0.834.80 6.04 0.79 24 0.84 4.84 5.34 0.91 48 0.59 3.43 5.00 0.69

Example 5

[0058] Oxidized white liquor was produced by causticizing 0.75 L ofgreen liquor with 53 g reburned lime, spiked with 1.5 g MnO₂. After 60min causticizing time at 95° C., air was sparged into the causticizedslurry at a rate of 0.55 L/min for 58 min. After oxidation, theresulting CaCO₃ lime mud with added MnO₂, was separated from theoxidized white liquor. Samples of the clarified white liquor were thenstored in a thermostated bath held at a desired temperature. At regulartime intervals, small samples of liquor were withdrawn for determinationof polysulphide concentration by UV spectrometry (PS_(UV)) and bygravimetry (PS_(GRAV)).

[0059] Example 5 illustrates the increase in the concentration ofPS_(UV) measured at 285 or 286 nm and PS_(VIS) measured at 416 nm andthe PS_(UV)/PS_(GR) and PS_(VIS)/PS_(GR) ratio when a freshly oxidizedwhite liquor is treated at 60° C. for up to 20 hours.

[0060] The data in the table in this example shows that by heattreatment at 60° C. for 20 hours, the PS_(UV)/PS_(GR) or PS₂₈₅/PS_(GR)ratio was changed from 0.44 to 0.60, and the PS_(VIS)/PS_(GR) orPS₄₁₆/PS_(GR) ratio increased from 0.18 to 0.40, while the gravimetricpolysulphide concentration was only decreased from 8.4 g/L to 7.5 g/L.The PS₄₁₆ concentration in the oxidized liquor doubled from 1.5 to 3.0gpl (as sulphur) during the 20 hours of storage at 60 C. As can be seenfrom FIG. 1, such an increase in the PS_(UV)/PS_(GR) or PS₂₈₅/PS_(GR)ratio will allow the yield increase from a given concentration ofpolysulphide to be increased from zero to approximately a third of thefull potential of that concentration. TABLE 5 Stability at 60° C. PScon- PS con- PS con- concentrat- concentrat- UV Vis Sample concentrat-ion (UV ion (Vis 285/ 416/ Time ion (gravi- 285) 416) Grav Grav (h)metry) g/L) g/L g/L) ratio ratio 0 8.4 3.7 1.5 0.44 0.18 1 8.3 3.8 1.60.46 0.19 3 8.1 3.8 1.8 0.47 0.22 16 7.9 4.3 2.7 0.54 0.34 20 7.5 4.53.0 0.60 0.40

Example 6

[0061] Example 6 illustrates the change in the ratio when the sameliquor is heat treated at 80° C. At this temperature the activation ofthe liquor, as measured by the change in ratio, is more rapid. Within 16hours the ratio has increased to a useful 89.

[0062] The data in the table in this example shows that by heattreatment at 80° C. for 20 hours, the PS_(UV)/PS_(GR) or PS₂₈₅/PS_(GR)ratio was changed from 0.44 to 0.93, and the PS_(VIS)/PS_(GR) orPS₄₁₆/PS_(GR) ratio increased from 0.18 to 0.86, while the gravimetricpolysulphide concentration was decreased from 8.4 g/L to 4.6 g/L. As canbe seen from FIG. 1, such an increase in PS_(UV)/PS_(GR) orPS₂₈₅/PS_(GR) ratio will allow the yield increase from a givenconcentration of polysulphide to by increased from zero to almost thefull potential of that concentration. TABLE 6 Stability at 80° C. PScon- PS con- PS con- concentrat- concentrat- UV Vis Sample concentrat-ion (UV ion (Vis 285/ 416/ Time ion (gravi- 285) 416) Grav Grav (h)metry) g/L) g/L g/L) ratio ratio 0 8.4 3.7 1.5 0.44 0.18 1 7.7 3.8 1.90.49 0.25 3 6.6 4.0 2.6 0.60 0.39 16 4.8 4.3 3.9 0.89 0.81 20 4.6 4.34.0 0.93 0.86

Example 7

[0063] Example 7 illustrates the change in the ratio when a similarliquor is heat treated at 70° C. At this temperature the activation ofthe liquor, as measured by the change in ratio, is less rapid than at80° C., but more rapid than 60° C. Within 20 hours, the PS_(UV)/PS_(GR)or PS₂₈₅/PS_(GR) ratio has increased to a useful 0.72 and thePS_(UV)/PS_(GR) or PS₄₁₆/PS_(GR) ratio increased from 0.21 to 0.57.Through 20 hours of heat treatment at 70 C, the PS₄₁₆ concentration wasmore than doubled from 1.8 to 3.8 gpl (as sulphur). TABLE 7 Stability at70° C. PS con- PS con- PS con- Sample centration centration centrationUV Vis Time (gravimetry) (UV 285) (Vis 416) 285/Grav 416/Grav (h) g/Lg/L g/L ratio ratio 0 8.5 4.0 1.8 0.47 0.21 1 8.7 4.0 1.8 0.46 0.21 38.5 4.1 2.2 0.48 0.26 5 8.0 4.2 2.3 0.52 0.29 16  6.6 4.7 3.5 0.71 0.5320  6.7 4.8 3.8 0.72 0.57

Example 8

[0064] Example 8 illustrates the change in the ratio when the sameliquor is heat treated at 90° C. At this temperature, the activation ofthe liquor, as measured by the change in ratio, is very rapid. Within 5hours, the PS_(UV)/PS_(GR) or PS_(UV)/PS_(GRAV) ratio has increased to auseful 0.84, and the PS_(VIS)/PS_(GR) or PS₄₁₆/PS_(GR) ratio increasedfrom 0.21 to 0.74, while the gravimetric polysulphide concentration wasdecreased from 8.5 g/L to 4.3 g/L. Longer times at this temperatureresulted in a lower PS_(GR), PS_(UV) and PS_(VIS) concentrations withonly a small gain in the ratios. TABLE 8 Stability at 90° C. PS con- PScon- PS con- Sample centration centration centration UV Vis Time(gravimetry) (UV 285) (Vis 416) 285/Grav 416/Grav (h) g/L g/L g/L ratioratio 0 8.5 4.0 1.8 0.47 0.21 1 6.9 3.7 2.2 0.53 0.31 3 4.8 3.6 2.9 0.740.61 5 4.3 3.6 3.2 0.84 0.74 16  3.8 3.3 3.1 0.86 0.80 20  3.6 3.3 3.10.91 0.84

EXAMPLE 9

[0065] Unclarified mill white liquor containing 100 g/L of lime mud wasoxidized with air in the presence of 2.0 g/L MnO₂ at 85-90° C. for 60minutes, cooled quickly to room temperature (20° C.) using a water bathand filtered to remove the lime mud and to give a clarified oxidizedwhite liquor. One portion of the oxidized white liquor was treated at70° C. for 20 hours. Another portion was stored at room temperature (20°C.) for 20 hours. The polysulphide concentrations in these two oxidizedwhite liquors were determined by gravimetry to be PS_(GR)=6.4 g/L and7.7 g/L, respectively, and by UV to be PS_(UV)=5.1 g/L and 3.8 g/L,respectively. The PS_(UV)/PS_(GR) of the oxidized white liquor treatedat 70° C. for 20 hours (OWL-70° C.) was thus 0.80 and thePS_(UV)/PS_(GR) of the oxidized white liquor stored at 20° C. for 20hours (OWL-20° C.) was 0.49. The same amounts of these two oxidizedwhite liquors were then used for the pulping of mixed softwood chips(50/50 black spruce and pine) in a micro-digester using 50 g (OD weight)of the wood chips in each of four stainless steel laboratory bombs. Acontrol Kraft cook using the white liquor (WL) was also carried out. Theliquor to wood ratio and the maximum cooking temperature were 4.5 to 1and 170° C., respectively. The PS_(GR) charges were 1.3 and 1.5% (onwood) for the cook using OWL-70° C. and the cook using OWL-20° C.,respectively. Each bomb was cooked to a certain H-factor. Uponcompletion of each cook, the pulp from each bomb was well washed andscreened through a laboratory flat screen plate (0.2 mm or 0.008″ slot).The screened pulp yields were measured by weighing the oven-driedscreened pulps and the permanganate numbers determined according toPAPTAC, Standard G. 17H. FIG. 3 shows that the PS cook using theheat-treated oxidized white liquor (OWL-70° C.) at a ratio ofPS_(UV)/PS_(GR)=0.80 gives a higher yield gain over the Kraft referencethan the cook using oxidized white liquor without the heat treatment(OWL-20° C.) at a ratio of PS_(UV)/PS_(GR)=0.49.

Example 10

[0066] Unclarified mill white liquor containing 100 g/L of lime mud wasoxidized with air in the presence of 2.0 g/L MnO₂ at 85-90° C. for 60minutes, cooled quickly to room temperature (20° C.) using a water-bathand filtered to remove the lime mud and to give a clarified oxidizedwhite liquor. One portion of the oxidized white liquor was treated at70° C. for 20 hours. Another portion was stored at room temperature (20°C.) for 20 hours. The polysulphide concentrations in these two oxidizedwhite liquors were determined by gravimetry to be PS_(GR)=6.0 g/L and7.6 g/L respectively. The same amounts of these two oxidized whiteliquors were then used for the pulping of maple chips in amicro-digester using 50 g (OD weight) of the wood chips in each of fourstainless steel laboratory bombs. A control Kraft cook using the whiteliquor (WL) was also carried out. The liquor to wood ratio and themaximum cooking temperature were 4.0 to 1 and 165° C., respectively. ThePS_(GR) charges were 1.1 and 1.4% (on wood) for the cook using OWL-70°C. and the cook using OWL-20° C., respectively. Each bomb was cooked toa certain H-factor. Upon completion of each cook, the pulp from eachbomb was well washed and screened through a laboratory flat screen plate(0.2 mm or 0.008″ slot). The screened pulp yields were measured byweighing the oven-dried screened pulps and the permanganate numbersdetermined according to PAPTAC, Standard G. 17H. FIG. 4 shows that thePS cook using the heat-treated oxidized white liquor (OWL-70° C.) againgives a higher yield gain over the Kraft reference than the cook usingoxidized white liquor without the heat treatment (OWL-20° C.).

Example 11

[0067] This example (through FIG. 5) summarizes the optimum storage timeneeded to maximize the PS_(UV) content of a polysulphide liquorgenerated by the oxidation of white liquor. The active polysulphideconcentration (PS₄₁₆) at a given storage time is described by the curvewhich increases with time. The temperature at which the liquor is heldin storage is described by the curve that decreases with time. FIG. 3shows that, at the lowest temperature evaluated (60° C.), a storage timeof 60 hours is needed to produce 6 g/L of active polysulphide from aliquor initially having a PS_(GR) concentration of 8.5 g/L. At thehighest temperature evaluated (103° C.) a storage time of 2 hours isneeded to produce 2.3 g/L of active polysulphide from the same liquor.

Example 12

[0068] In this example, a polysulphide liquor was made with MnO₂ butwithout lime mud. A synthetic white liquor was prepared from sodiumhydroxide and sodium sulphide. A sample (750 mL) of this white liquorwas oxidized using air at 450 ml/min with 0.4 grams of a commercialgrade MnO₂ (0.53 g/L MnO₂). The composition of the synthetic whiteliquor and the product oxidized liquor are illustrated in Table 9. Table10 shows that heat treatment at 77° C. for 16.5 hours of this type ofoxidized liquor is effective in increasing the PS_(UV)/P_(GR) ratio from0.46 to 0.97 and increasing the PS₄₁₆ concentration in the oxidizedliquor from 1.4 to 3.3 gpl (as sulphur). TABLE 9 Composition of liquorsused in this example. Synthetic White 60 minutes Liquor Oxidation Na₂S,g/L as S 18.11 11.16 PS, g/L as S  0.24  7.14 % Selectivity n/a 99

[0069] TABLE 10 Effect of thermal treatment (storage at 77 C. for 16.5hours) with a polysulphide liquor generated with MnO₂ in the absence oflime mud. UV 285 nm 416 nm Gravimetric 285/grav PS, g/L S PS, g/L S PS,g/L S ratio Before Thermal 3.3 1.4 7.14 0.46 treatment After Thermal 3.63.3 3.65 0.97 treatment

Example 13

[0070] In this example, a polysulphide liquor was made with awet-proofed activated carbon catalyst. Wet proofing was done by sprayinga dry film lubricant (TFE in Freon-trademark) on activated carbon(50-200 mesh from Fisher Scientific Co. Ltd.). The resulting paste wasdried in the fume hood under a flow of nitrogen.

[0071] Table 11 shows that heat treatment at 65° C., or aging at 25° C.,over 60 hours of this type of oxidized liquor increases the activepolysulphide from 2.15 g/L to 4.55-5.68 g/L.

[0072] The oxidation was done at room temperature by adding 5 g ofwet-proofed carbon to about 300 mL of artificial white liquor pre-heatedto 85° C. in a 500 mL beaker. Oxidation was done for about 15 h byletting air diffuse through the floating carbon into the white liquor.Samples of the oxidized liquor were then analyzed by gravimetry and byUV spectrophotometry, just after production (fresh) and then afterstorage for 60 h at 25° C. Another aliquot of the fresh oxidized liquorwas also stored at 65° C. for 60 h and then analyzed again.

[0073] The effect of liquor aging on the change in active (PS^(act)),inactive (PS^(inact)) and total (PS^(tot)) concentrations ofpolysulphides is presented in Table 11. TABLE 11 Effect of thermaltreatment with a polysulphide liquor generated using a wet-proofedactivated carbon catalyst PS concentrations (g/L, as S) MOXY-type liquorPS^(act) PS^(inact) PS^(tot) Fresh 2.15 10.39 12.54 Aged 60 h at 25° C.4.55 3.07 7.62 Aged 60 h at 65° C. 5.68 3.58 9.20

LITERATURE REFERENCES

[0074] Alfredsson, B., Samuelson, O. and Sandstig, B. Carboxyl endgroups in sulfate and polysulphide pulps. Svensk Papperstidn. 66(18):703(1963).

[0075] Clayton, D. W. and Sakai, A. Multi-stage polysulphide pulpingprocesses. Part I. Basic ideas and low-temperature impregnation studieson black spruce heartwood. Pulp Pap. Mag. Can. 68(12):619 (1967).

[0076] Dorris, G. M. Process of producing Kraft pulping liquor by theoxidation of white liquor in the presence of lime mud. U.S. Pat. No.5,082,526, Paprican (1992).

[0077] Hartler, N. Sulphate cooking with the addition of reducingagents. Part 1. Preliminary report on the addition of sodiumborohydride. Svensk Papperstidn. 62(13):467 (1959).

[0078] Holton, H. H. Delignification of lignocellulosic material with analkaline liquor in the presence of a cyclic keto compound. U.S. Pat. No.4,012,280, C.I.L. (1977).

[0079] Landmark, P. A., Kleppe, P. J. and Johnsen, K. Cooking liquoroxidation and improved cooking technique in polysuphide pulping. Tappi J48(5):56 (1965).

[0080] Pettersson, S. E. and Rydholm, S. A. Hemicelluloses and paperproperties of birch pulps. Part 3. Svensk Papperstidn. 64(1):4 (1961).

[0081] Sanyer, N. and Laundrie, J. F. Factors affecting yield increaseand fiber quality in polysulphide pulping of loblolly pine, othersoftwoods, and red oak. Tappi J. 47(10):640 (1964).

[0082] Smith, G. C. and Sanders, F. W. Production of polysulphide withPTFE coated catalyst. U.S. Pat. No. 4,024,229 (1977).

[0083] Teder, A. Some aspects of the chemistry of polysulphide pulping.Svensk Papperstidn. 72(9):294 (1969).

[0084] *Tench, L., Uloth, V., Dorris, G., Hornsey, D., and Munro, F.Mill Scale Implementation of Paprican's Process for Polysulpide LiquorProduction in Kraft Mill's Causticizers, Part 1: Batch Trials andProcess Optimization. Tappi J. 82 (10): 120 (1999).

[0085] Uloth, V., Dorris, G., Thring, R., Hogikyan, R., Tench, L., andAyton, J. Production of Polysulphide Liquor in a Kraft Mill'sCausticizers. Tappi J. 80(10): 223 (1997).

1. A method which comprises exposing an oxidized white liquor producedby oxidizing a white liquor with oxygen containing gas in the presenceof lime mud, manganese dioxide or both, to a controlled temperatureeffective to increase the concentration of PS_(UV) measured at 285 or286 nm or PS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) orPS_(VIS)/PS_(GR) ratio of polysulphide in the oxidized white liquor. 2.A method according to claim 1, wherein said temperature is 20° C. to 95°C. and said exposing is for a time up to 72 hours.
 3. A method accordingto claim 2, wherein said oxidized white liquor is maintained at atemperature of 50° C. to 90° C. for 1 to 48 hours.
 4. A method ofincreasing the concentration of PS_(UV) measured at 285 or 286 nm orPS_(VIS) measured at 416 nm and the PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR)ratio of a polysulphide liquor generated by oxidation of white liquor inthe presence of lime mud, manganese dioxide or both, said methodcomprising heating or cooling if necessary and then storing saidpolysulphide liquor at a temperature between 20° C. and 95° C. for atime of up to 72 hours.
 5. A method according to claim 4, wherein saidpolysulphide liquor is maintained at a temperature of 50° C. to 90° C.for 1 to 48 hours.
 6. A method of increasing the yield of pulp in Kraftpulping with a white liquor containing polysulphide comprising: i)oxidizing a white liquor with oxygen containing gas in the presence oflime mud, manganese dioxide or both to produce an oxidized white liquorcontaining polysulphide, ii) heating or cooling if necessary and thenstoring said oxidized white liquor to increase the concentration ofPS_(UV) measured at 285 or 286 nm or PS_(VIS) measured at 416 nm and thePS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio of polysulphide in theoxidized white liquor, and in a subsequent step: iii) delignifying pulpwith the oxidized white liquor from step ii).
 7. A method according toclaim 6, wherein said oxidizing in step i) is in the presence of limemud and a catalytic amount of manganese dioxide, and including a stepof: separating said oxidized white liquor produced in step i) from saidlime mud and manganese dioxide prior to said heating or cooling ifnecessary and storing in step ii).
 8. A method according to claim 7,wherein step ii) comprises heating or cooling if necessary said oxidizedwhite liquor to a temperature of 20° C. to 95° C. for a time up to 72hours.
 9. A method according to claim 8, wherein said temperature is 50to 90° C. and said time is 1 to 48 hours.
 10. A method according toclaim 8, including prior to step i): causticizing a green liquor withlime in the presence of manganese dioxide to produce said white liquorand lime mud containing said manganese dioxide.
 11. A method ofproducing an oxidized white liquor containing polysulphide comprising:i) oxidizing a white liquor with oxygen containing gas in the presenceof lime mud, manganese dioxide or both to produce an oxidized whiteliquor containing polysulphide and having a first PS_(UV) concentrationmeasured at 285 or 286 nm or PS_(VIS) concentration measured at 416 nmor PS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio, and ii) heating or coolingif necessary and then storing said oxidized white liquor to produce anoxidized white liquor having a second PS_(UV) concentration measured at285 or 286 nm or PS_(VIS) concentration measured at 416 nm orPS_(UV)/PS_(GR) or PS_(VIS)/PS_(GR) ratio, wherein said secondconcentration or ratio is greater than said first ratio.
 12. A methodaccording to claim 11, wherein step ii) comprises heating or cooling ifnecessary said oxidized white liquor to a temperature of 20° C. to 95°C. and storing said oxidized white liquor for a time up to 72 hours. 13.A method according to claim 12, wherein said temperature is 50 to 90° C.and said time is 1 to 48 hours.
 14. A method according to claim 13,including prior to step i): causticizing a green liquor with lime in thepresence of manganese dioxide to produce said white liquor and lime mudcontaining said manganese dioxide.